This invention relates to a device for purifying exhaust gases from internal combustion engines, and more particularly to a device for use in a vehicle for purifying engine exhaust gases by a catalytic reaction so as to remove nitrogen oxides and combustible components in the exhaust gases.
It is commonly known that exhaust gases discharged from an internal combustion engine contain harmful components, for example, oxides such as nitrogen oxides NO.sub.x and combustible matters or products of incomplete combustion such as hydrocarbons and carbon monoxide, and these harmful components give rise to a serious social problem as they pollute the atmosphere. It is necessary to subject exhaust gases to reduction in order to remove NO.sub.x, and it is necessary to subject exhaust gases to oxidation in order to remove HC and CO. Various exhaust gas purification apparatus have been studied and proposed hitherto in an effort to remove these harmful components. Catalysts are commonly employed for attaining smooth and reliable reduction and oxidation of these harmful exhaust gaseous components and improving the efficiency of exhaust gas purification.
A so-called overall purifying device for purifying exhaust gases from an internal combustion engine of a vehicle has been proposed in an attempt to remove all of the harmful components including NO.sub.x, HC and CO contained in the engine exhaust gases. The proposed device comprises a catalytic reduction unit disposed in the vicinity of the engine for removing NO.sub.x by reduction and a catalytic oxidation unit disposed at a position rearward of the engine, for example, beneath the floor of the vehicle for removing HC and CO by oxidation. In order that purification of the exhaust gases can be more completely attained, the efficiency of exhaust gas purification must be improved in such various engine operating conditions as starting, acceleration and deceleration where the concentration of harmful components is especially high. To this end, it is necessary to supply secondary air and control, for example, the flow properties of exhaust gases depending on the operating condition of the engine. However, provision of such a plurality of independent catalytic units is undesirable in view of the complexity of structure. Further, it is necessary to maintain the temperature of exhaust gases introduced into the catalytic units at a sufficiently high level so as to improve the conversion efficiency during starting of the engine. From this standpoint too, it is desirable to dispose the catalytic units closer to the engine.